Climate change could — if the worst predictions of scientists come true — lead to a drop of between 20 and 37 per cent in China's yield of rice, wheat and maize over the next 20 to 80 years, according to a report published yesterday (13 September) by the Chinese and British governments last week.

But the report also predicts that cotton yields are likely to increase by about 40 per cent over this period because of global warming.

According to the UK environment minister, Elliot Morley, the study shows that "unless greenhouse gas emissions are reduced, climate change could seriously affect agricultural production in China".

In general, higher concentrations of atmospheric carbon dioxide — the main greenhouse gas — lead to increased crop yields, a phenomenon called 'carbon dioxide fertilisation'. But this effect is variable, and depends on other factors, such as the availability of water and nutrients. Meanwhile higher temperatures tend to result in lower yields because they cause plants to mature too quickly.

The study used two scenarios from the Intergovernmental Panel on Climate Change's (IPCC) Special Report on Emissions Scenarios. In one, calculations are based on the assumption that the global human population grows continuously, and there is a medium-high increase in greenhouse gas emissions. The second scenario incorporates regional sustainable development, a slower population increase and a medium-low increase in greenhouse gases.

The researchers also modelled the carbon fertilisation effect, and created a worst-case scenario in which the effect was absent. Under such conditions, yields of rice, maize and wheat were predicted to fall, regardless of which IPCC scenario was used.

When the carbon dioxide fertilisation effect was included in the model, yields of wheat increased in each scenario, but those of rice and maize depended on which scenario was used and whether the crops were irrigated or rain fed.

The study says, however, that an optimal carbon dioxide fertilisation effect is unlikely in reality. Although the modelling examined variations in climate, soil and carbon dioxide in the atmosphere, it did not consider water availability, which influences the potential for a fertilisation effect. Crop pests and diseases were also not considered.

The £400,000 study was conducted by the UK Department for the Environment, Food and Rural Affairs and China's Ministry of Science and Technology (MOST). During the research, five Chinese scientists spent time developing climate models with UK researchers at the Hadley Centre for Climate Prediction and Research in Exeter.

The models developed predict that China will experience more extreme climatic events including warmer summers and more days with heavy rainfall. The study also suggests that average temperatures across China will rise by 3-4°C by the end of the 21st century.

These findings follow the publication in June of research linking increased night-time temperatures associated with global warming to decreasing rice yields in the Philippines (see Global warming 'threatens rice yields').

The China-UK collaboration was agreed in 2001 and is set to continue into a second phase, which will begin in 2005. The next stage of the study will consider the effect of climate change on water supply and how this will affect crop yields.

"I welcome the fact that the project has helped to increase closer working relations on climate change as well as developing mutual understanding between scientists from the two nations," says Morley. "I look forward to developing our collaboration further in the second phase of the project."